Before a case-ending beginning with y or bh (literally a consonant of the yañ pratyahāra) the final of a Nominal-stem is lengthened.,
The whole of the phrase अतो दीर्घो यञि is understood here. Thus, वृक्षाय, प्लक्षाय, वृक्षाभ्याम्, प्लक्षाभ्याम् ॥ But अग्निभ्याम् where the stem ends in इ, and वृक्षस्य प्लक्षस्य where the affix does not begin with a यञ् consonant.,